In a typical installation, telephone service is provided between a central office and a customer's premises via a pair of wires, commonly referred to as "tip" and "ring". The tip and ring wires are twisted together and exit from the central office within a cable which typically includes a large of number of twisted wire pairs, enclosed in a metal jacket or sheath connected to ground. As the customers' premises vary in distance from the central office, so too is there substantial variation in the length of the wire pairs extending from the central office to the customer locations.
There has been increasing demand for transmission of digital data over conventional twisted pair telephone lines. One type of digital transmission service typically provided over twisted pair lines is referred to as ISDN (integrated services digital network). Reliable ISDN service cannot be provided over a twisted wire pair that has more than 42 dB of insertion loss, measured with a throughput frequency of 40 kHz. If a wire pair has an overall length of 18,000 feet, and is implemented according to standard practice with 10,500 feet of 26 AWG wire and 7,500 feet of 24 AWG wire, such a wire pair would have an insertion loss of about 42.6 dB at 70.degree. F. If such a wire pair were installed in an aerial plant cable and exposed to sun loading in the summer, the insertion loss would be in excess of 42.6 dB. The inventor believes that a prudent policy would consider a wire pair qualified for ISDN service only if the length of the wire pair does not exceed 15,000 feet.
It would therefore be desirable to pre-qualify wire pairs for digital transmission service by a test procedure which measures the length of the wire pair from the central office to the customer premises.
It is known to attempt to measure the length of wire pairs from the central office using conventional test equipment such as the Model 105A remote test unit (RTU) marketed by Harris Corporation. When there is no terminal equipment having a ringer installed at the end of the wire pair, the Harris RTU can determine the length of the wire pair by measuring tip-to-ring (T-R) capacitance using a 30 Hz test signal. (Use of a 30 Hz test signal reflects teachings of U.S. Pat. Nos. 4,611,101 (Walter et al.); 4,113,998 (Ashdown et al.); and 4,139,745 (Ashdown et al.). These patents indicate that, in general, higher test frequencies produce less reliable results and are therefore undesirable.) If terminal equipment having a ringer is installed at the end of the wire pair, then tip-to-ground (T-G) and ring-to-ground (R-G) capacitance are measured to determine the length of the wire pair. However, there is a risk of substantial inaccuracy in all these measurements.
First of all, it often is not known a priori whether or not terminal equipment with a ringer is installed on the wire pair. If a ringer is present, there is a large change in the T-R capacitance measured at the central office, and therefore the T-R capacitance cannot be considered as a reliable indication of the wire pair length. The T-G and R-G capacitance measurements are also subject to significant unreliability, since the cable sheath may not be continuous. For example, it sometimes occurs that there is a failure to splice the sheath at the site of cable splices, or the sheath may experience corrosion over time which results in discontinuity. As a result, the length of the sheath indicated by the T-G and R-G capacitance measurements may be substantially different from the actual length of the wire pair in question.